In the field of logging (e.g. wireline logging, logging while drilling (LWD) and measurement while drilling (MWD)), nuclear magnetic resonance (NMR) tools have been used to explore geographic formations based on the magnetic interactions with subsurface material. Some downhole NMR tools include a magnet assembly that produces a static magnetic field, and a coil assembly that generates radio frequency (RF) control signals and detects magnetic resonance phenomena in the subsurface material. Properties of the subsurface material can be identified from the detected phenomena.
When using NMR-based tools, it can be difficult to achieve a satisfactory signal-to-noise ratio (SNR) based on the natural polarization of the spins. In addition, the ex situ (i.e., outward looking) nature of downhole NMR tools means that the excitation field naturally falls off in strength over distance from the tool. Thus, the further the antenna is from the sensitive volume, the lower the amplitude of the corresponding received signal.